Generator rotors have a large diameter cylindrical body from which extends at both ends a smaller diameter shaft. The rotor body has a series of longitudinal slots cut deep into its outer circumference. In these slots are inserted field windings that extend the length of the rotor body. There are wedges in the slots that hold the windings in place against centrifugal forces exerted when the rotor rotates. These wedges are above the windings and may protrude radially outward of the outer circumferential surface of the rotor body.
The end turn portions of the windings extend axially out beyond each end of the rotor body. These end turns electrically connect the longitudinal section of a winding in one slot with a similar winding section in another slot. As the rotor spins, the end turns are thrust radially outward by centrifugal force. This radial movement of the end turns is confined by retaining rings that enclose the end turns and are placed over the ends of the rotor body. Centrifugal forces cause the end turns to press firmly against the inside surface of the retaining rings. In this way, the retaining rings hold the end turns in place during operation of the generator.
Retaining rings are usually attached to the ends of the rotor body by shrink fitting. The rim of one end of the retaining ring is shrink fitted tightly around a circumferential lip on the end of the rotor body. In addition, locking keys securely hold the retaining rings onto the rotor body to prevent axial movement of the rings. These keys fit in opposing grooves in the retaining ring and in both the rotor teeth and wedges. Without these keys, thermal expansion of the field coils and the retaining rings may cause the retaining rings to slide axially off the rotor.
In some applications, high frequency currents exist on the surface of the rotor body and retaining rings. For example, when the generator is used in conjunction with a load commutated inverter LCI), cycloconverter (CCV) or other non-linear load, eddy currents are induced on the surface of the rotor. These rotor eddy currents are the results of harmonics of the input and/or output currents of the LCI and CCV devices. The currents have high frequencies and, thus, primarily reside at and near the surface of the rotor and retaining rings.
Losses due to these eddy currents in the rotor result in undesirable I.sup.2 R (Joulean) heating. Accordingly, low resistance current paths are needed to reduce eddy current losses and thus minimize heating. The rotor body and retaining rings, typically made of high strength steel alloys, are not themselves good electrical conductors. The relatively high electrical resistance of the retaining rings and rotor body to the eddy currents will cause high losses and heating. To avoid these losses, eddy shields cover the outer surfaces of the rotor body and retaining rings to provide a low resistance electrical path for surface eddy currents. The shields reduce losses of eddy currents and can prevent localized heating of the rotor body and retaining rings.
An eddy shield is commonly a thin copper layer applied to the outer surfaces of the rotor body and retaining rings. The shield can be a jacket or a cladding. In the alternative, an eddy current shield for the rotor can be provided by specially configured wedges made of conductive chromium-copper or other alloys that have wings that overhang the rotor teeth adjacent the wedge. These wedges are disclosed in the copending application entitled "Reducing Harmonic Losses in Dynamoelectric Machine Rotors" referenced above and incorporated by reference.
While eddy shields substantially reduce current losses, they do not reduce the magnitude of the eddy current. The current shields merely provide low resistance paths for the currents. Any interruption or point of high resistance in these current paths will cause additional loss and localized heating. It is desirable to provide a reliable low resistance electrical connection between the rotor body and retaining rings. As discussed below, prior couplings between rotors and retaining rings do not provide good electrical connections for high-frequency currents.
One example of a eddy current junction between the rotor and retaining ring is shown in U.S. Pat. No. 4,275,324. This patent discloses a radial extension on the wedge that rises to meet the eddy current shield on the retaining ring. The patent does not clearly disclose how the contact is made between the extension on the wedge and the eddy current shield to the retaining ring, or how the contact is maintained at all rotor operating speeds.
The present invention provides a reliable electrical contact for eddy currents from the rotor surface to the retaining ring. An electrical contact is provided between the surface of the rotor and the retaining rings at all operating speeds. This contact is a multilam or belleville spring in a groove mounted in the surface of the wedges and rotor teeth. The spring is biased against the inner surface of the retaining ring. This spring provides an electrical path for eddy currents.
It is an objective of this invention to provide a reliable electrical junction between a rotor and a retaining ring for high-frequency eddy currents. In particular, it is an object of this invention to provide such a junction by means of a multilam or belleville spring mounted between the outer surface of a wedge and the inner surface of the overlapping end of the retaining ring.